1. Field of the Invention
This invention relates to a laser Doppler velocimeter (LDV) incorporating an improved focusing system. More particularly, it relates to such an LDV in which a constant laser beam waist size and position at the crossing point of the laser beams is maintained when the LDV is focused at different points.
2. Description of the Prior Art
LDVs have proven to be very valuable tools for measuring velocity characteristics of moving fluids, particularly for measuring air flow around airfoils and similar structures in wind tunnels. The basic principle of operation is that coherent laser light scattered from particulate matter in a fluid moving with a particular velocity will be Doppler shifted by an amount determined by the laser wavelength and the index of refraction of the scattering medium. It is often desirable to measure different LDV parameters simultaneously to give a more complete description of the fluid flowfield being studied. Because of the high data rates generated by LDVs and the amount of signal processing of the LDV data required to characterize the fluid flow, LDVs place rather severe demands on data processing systems to which they are connected and to data interfaces between the LDVs and the data processing systems. LDVs are described, for example, in the following issued U.S. Patents: U.S. Pat. No. 3,860,342, issued Jan. 14, 1975 to Orloff et al.; U.S. Pat. No. 3,895,872, issued July 22, 1975 to Dandliker et al.; U.S. Pat. No. 3,897,152, issued July 29, 1975 to Farmer et al.,; U.S. Pat. No. 4,063,814, issued Dec. 20, 1977 to Rhodes; U.S. Pat. No. 4,148,585, issued Apr. 10, 1979 to Bargeron et al.; U.S. Pat. No. 4,167,329, issued Sept. 11, 1979 to Jelalian et al. and U.S. Pat. No. 4,346,990, issued Aug. 31, 1982 to Rhodes. The state of the art in LDVs is further indicated in Durst et al., "Influence of Gaussian Beam Properties on Laser Doppler Signals", Applied Optics, 18, No. 4, pp. 516-524, Feb. 15, 1979.
In particular, the Rhodes U.S. Pat. No. 4,346,990 provides an ingenious geometric optics solution to the need for rapid scanning along an optical axis to produce high speed sampling of flows being characterized with the LDV apparatus. However, the fixed afocal lens and movable scanning lens technique there described does not address the diffraction optics aspects of the problem.
The beams from many lasers, including those most used in velocimetry, have a Gaussian intensity profile. The narrowest place, or waist, of a Gaussian beam corresponds in a certain sense to the focus of a typical geometric optics beam. For example, if the narrowest parts of a geometric optics beam are at equal distances in front of and behind a lens, they are each at a distance of 2f from the lens of focal length f and are the same size. If the waist of a Gaussian laser beam is placed one focal length in front of a lens, the lens will form a new beam waist one focal length behind the lens and not necessarily the same size as the input waist.
In the apparatus of the Rhodes U.S. Pat. No. 4,346,990, two beams cross, always at the same angle, and their crossing volume forms the sensitive volume of the velocimeter. But, if each of these beams does not have a beam waist located at their intersection, the interference fringes formed by their crossing will not be parallel. As a result, beam accuracy is affected, because the beam waists will not remain at the beam intersection as the velocimeter is scanned, resulting in degradation of the measurement.
Further, the beam waists that should cross to form the sensitive volume do not remain the same size as the system is scanned. It is often undesirable for the focus spot diameters, i.e., the beam waists at the crossing, to become larger. The light intensity, and thus the light scattered from a dust particle, goes down as the inverse square of the focus spot diameter. Similarly, it is often undesirable for the focus spot diameter to become much smaller. This reduces the number of fringes that can be counted during passage of a particle through the test volume. These volume changes can also influence the collection of data so that counting rates would be different in different parts of the fluid stream. The optimum bias setting for noise rejection is different for different parts of the scan with different waist volumes.
The other above-identified patents disclose a variety of techniques for focusing parallel LDV beams at points in space. These techniques provide a scan by either moving a focusing element or by employing a focusing element of variable focal length. However, such techniques inherently cannot maintain constant crossbeam angle and focal volume dimensions. If the crossbeam angle and focal volume are not maintained constant, the Doppler shift in the scattered light is dependent on the focal volume location as well as the velocity of the scattering particle and require calibration of equipment for each focal position.